Optical scanners are used in a wide range of applications. Some scanners use a rotating mirror to scan a beam.
For example, PCT International Publication WO 2012/020380, whose disclosure is incorporated herein by reference, describes apparatus for mapping, which includes an illumination module. This module includes a radiation source, which is configured to emit a beam of radiation, and a scanner, which is configured to receive and scan the beam over a selected angular range. Illumination optics are configured to project the scanned beam so as to create a pattern of spots extending over a region of interest. An imaging module is configured to capture an image of the pattern that is projected onto an object in the region of interest. A processor is configured to process the image in order to construct a three-dimensional (3D) map of the object.
In one of the embodiments described in this PCT publication, an illumination module comprises one or more beam sensors, such as photodiodes, which are coupled to a processor. These sensors are positioned at a selected angle or angles within the angular range that is scanned by a mirror so as to receive the scanned beam periodically and thus verify that the scanner is operating.
U.S. Patent Application Publication 2013/0207970, whose disclosure is incorporated herein by reference, describes depth engines that generate 3D mapping data by measuring the time of flight (TOF) of a scanning beam. A light transmitter, such as a laser, directs short pulses of light toward a scanning mirror, which scans the light beam over a scene of interest. The scanner may comprise a micromirror produced using microelectromechanical system (MEMS) technology. A receiver, such as a sensitive, high-speed photodiode (for example, an avalanche photodiode) receives light returned from the scene via the same scanning mirror. Processing circuitry measures the time delay between the transmitted and received light pulses at each point in the scan. This delay is indicative of the distance traveled by the light beam, and hence of the depth of the object at the point. The processing circuitry uses the depth data thus extracted in producing a 3D map of the scene.
U.S. Patent Application Publication 2013/0250387, whose disclosure is incorporated herein by reference, describes scanning apparatus that includes a transmitter, which is configured to emit a beam comprising pulses of light, and a scanning mirror, which is configured to scan the beam over a scene. A receiver is configured to receive the light reflected from the scene and to generate an output indicative of the pulses returned from the scene. A grating is formed on an optical surface in the apparatus and is configured to diffract a portion of the beam at a predetermined angle, so as to cause the diffracted portion to be returned from the scanning mirror to the receiver. A controller is coupled to process the output of the receiver so as to detect the diffracted portion and to monitor a scan of the mirror responsively thereto.